With King Edward’s death of illness, Danes under King Anlaf invade Northumbria and Prince Ælfweard moves on the throne with the backing of the ealdormen of Wessex and mercenaries, forcing Queen Eadgifu and Prince Edmund to flee to Uhtred for refuge. In Bebbanburg, Ingrith prophesizes that “seven kings must die and the woman you love” before England is united while Uhtred, now an old man, dismisses the prophecy as he isn’t a king and has no wife. Uhtred gathers his friends to move on Ælfweard before he can cement his power so as to let Aethelstan take the throne, leaving his son Osbert to train Edmund. Aethelstan has grown very religious and taken Lord Ingilmunder as his advisor who uses the Bible to help dictate the prince’s actions.

In Aegelesburg, Uhtred and his men pose as mercenaries looking to fight for Ælfweard, only to learn that he plans to lure Aethelstan into a trap. However, Aethelstan anticipates and outwits his brother’s trap while Uhtred takes control of the town’s gates, allowing Uhtred to convince Ælfweard to surrender peacefully. Convinced that he has been chosen by God to be the rightful heir, Aethelstan kills his brother in cold blood rather than allowing him to go into exile and has his men slaughter Ælfweard’s men. Uhtred manages to convince Aethelstan to stand down against the wishes of Ingilmunder, but he refuses to kneel before Aethelstan until he’s officially crowned at Winchester.

However, Uhtred doesn’t attend Aethelstan’s coronation, wishing for Aethelstan to prove that he’s not a tyrant before swearing loyalty to him. Ingilmunder, who is secretly Aethelstan’s lover, uses Aethelstan’s fear of being judged poorly by God for his homosexuality to urge the new king to revive and expand upon Alfred’s dream of a united England while Uhtred grows closer with Eadgifu. Aethelstan claims Eoferwic for himself and demands that all kings of Britan, both from the mainland and outlying islands, pay tribute to him and begins planning a forced spread of Christianity, taking the sons of the kings hostage. Deducing the true nature of Aethelstan’s relationship with Ingilmunder, Uhtred attempts to convince him that he is being manipulated so that Ingilmunder can gain more power for himself, but Aethelstan refuses to listen and declares Uhtred King of Northumbria with a warning about Anlaf’s threat.

Ingilmunder secretly declares Uhtred an enemy of God in an attempt to have him killed while using the threat of Anlaf to attempt to take Bebbanburg without a fight. Aldhelm warns Uhtred, causing Aethelstan to hang him for treason, while Uhtred intercepts and captures Ingilmunder and offers to trade him for peace with England and a guarantee that his people won’t be forced to convert to Christianity. Taking advantage of the growing discord, Anlaf gathers the kings of Man, Shetland, Orkney and Strathclyde to move on England. Ingilmunder manages to convince one of Uhtred’s Christian servants to lure him into a trap, forcing Osbert to surrender Bebbanburg. However, unable to bring himself to kill Uhtred, Aethelstan sends him into exile, threatening to kill Osbert should he return while Eadgifu is sent to a convent and Osbert and Edmund are sent to Ingilmunder’s garrison at the Wirral Peninsula.

Aethelstan embarks upon an invasion of Scotland, prompting the previously reluctant Constantin to ally with Anlaf. Uhtred is captured by the Danes who ask him to assassinate Aethelstan to avoid war while Ingilmunder is revealed to be a Danish spy working for Anlaf. After failing to convince Aethelstan of Ingilmunder’s treachery, Uhtred returns to Bebbanberg to find it abandoned aside from Eadgifu and Ingrith and the other residents trapped by Anlaf’s men in a nearby cave where they’ve suffocated, fulfilling part of the prophecy. At the same time, Anlaf’s army lands unopposed at Wirral where Ingilmunder slaughters his own garrison for refusing to change sides.

Arriving at Wirral in search of their sons, Uhtred and Eadgifu find a remorseful Aethelstan, who had come in search of the truth after speaking with Uhtred, along with Osbert and Edmund who had managed to escape the massacre with information on where the six Danish armies are gathering at Brunanburh. Recognizing that he likely faces a losing battle, Aethelstan orders Uhtred and his allies away so that they will not fall with him, but Uhtred comes to the conclusion that their destiny is not to form England but to form the English as a united people.

During the Battle of Brunanburh, Uhtred leads the severely outnumbered army of England against the Danish army. When the Danes seemingly have the upper hand, Uhtred’s tactic of a surprise flanking attack led by Osbert turns the tide of battle, leading to the deaths of most of the Danes, including the five sons and heirs of the kings of Scotland, Man, Shetland, Orkney and Strathclyde, and Anlaf’s daughter. Recognizing the futility of further conflict, Anlaf’s disheartened allies abandon him. While attempting to rush to Aethelstan’s aid, Uhtred is gravely wounded. Aethelstan captures Ingilmunder who admits that he truly did have feelings for the young king, but Ingilmunder’s loyalty to his people was stronger. For his crimes, Aethelstan has Ingilmunder executed without a weapon in his hand so that he won’t make it into Valhalla.

At Bebbanberg, Uhtred, Aethelstan and Finan conclude that the deaths of Edward and the five heirs – five kings who will never be crowned – fulfill part of the prophecy that seven kings must die while Uhtred speculates that he himself is the seventh and asks only to be honored for standing by his oath rather than being part of history. Uhtred agrees to swear Northumbria to Aethelstan to form a united England on the condition that Aethelstan swear an oath never to marry and have heirs so that Edmund can take the throne unchallenged when Aethelstan dies. Aethelstan accepts and Uhtred swears allegiance to him, making Aethelstan king of Wessex, Mercia, East Anglia and Northumbria, fulfilling Alfred’s dream of a united England at last. Uhtred experiences a vision of Valhalla where he sees those that he’s lost along the way, including Brida and Ragnar, waiting for him while his friends urge him not to give up just yet, leaving Uhtred standing indecisively between the living and the dead.

In a voiceover, Finan states that the Saxon Chronicles do not state whether Uhtred survived or not, but those who knew him recognize Uhtred as the greatest warrior of their age and a man who made a kingdom. A title card reveals that Aethelstan ruled for fifteen years and he is considered to be the first and greatest king of medieval England while the Danes continued to invade England until its eventual conquest by their Norman descendants in 1066, yet the fortress at Bebbanburg still stands. Footage is shown of people visiting the present day Bamburgh Castle which is what Bebbanburg eventually became.